Induction motor torque control system

ABSTRACT

An induction motor torque control system is provided for resolving an excitation current command and a secondary current command into an excitation flux vector component and electromotive force-direction (torque direction) vector component, respectively, based on a torque command for the motor and an excitation flux command, decided for the motor, in such a manner that a linear output torque is obtained in response to these commands. A primary current command for the motor is obtained by combining these vector values.

BACKGROUND OF THE INVENTION

This invention relates to a system for controlling the torque of an induction motor.

Recently, vector control generally has been employed for controlling the velocity of induction motors. When controlling the velocity of an induction motor in accordance with such vector control, a torque command, excitation current, secondary current and slip frequency become non-linear due to such effects as the secondary leakage impedance and core loss of the motor in cases where velocity control is performed up to a region of high rotational velocity, as in the spindle motor of a machine tool, or the like, or in cases where control is performed to weaken excitation in accordance with the torque command at a constant rpm. As a result, the output torque also becomes non-linear with respect to the torque command and an accurate torque command cannot be produced.

A conventional approach for dealing with this is to use external equipment such as a torque sensor to measure motor torque as a means for accurately ascertaining the output torque of the motor. A problem that results is the high cost of equipment.

SUMMARY OF THE INVENTION

The present invention solves the aforementioned problem of the prior art, and an object thereof is to provide an induction motor torque control system in which output torque is linearly controlled in response to a torque command without requiring special torque measurement equipment.

A system for controlling the torque of an induction motor according to the present invention includes: means for deciding a torque command for an induction motor and an excitation flux command from an excitation frequency; means for deciding an excitation flux component of excitation current from the excitation flux; means for deciding a component, in the direction of an electromotive force, of the excitation current from the excitation flux and excitation frequency; means for deciding a component, in the direction of the electromotive force, of a secondary current of the motor from the torque command and excitation flux command; means for deciding a slip frequency from the component, in the direction of the electromotive force, of the secondary current and the excitation flux command; means for deciding an excitation flux component of the secondary current, from the component in the direction of the electromotive force, of the secondary current and from the slip frequency; and means for obtaining an excitation flux component, and a component in the direction of the electromotive force of the primary current of the motor, a the excitation flux component and from component in the direction of the electromotive force of the excitation current and secondary current, respectively, and for limiting a primary current command for the motor upon combining these components.

Accordingly, by obtaining a slip frequency command which will cause the required load current to flow, the present invention enables the torque command itself to be substituted for the output torque in order to linearly control the motor torque, this being performed without requiring special torque measurement equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the interconnection of devices in an embodiment of the present invention;

FIG. 2 is a flowchart illustrating a torque control operation of the present invention;

FIG. 3 is a schematic diagram illustrating an equivalent circuit of an induction motor; and

FIGS. 4 and 5 are vector diagrams showing the relationship of various characteristics of an induction motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described in detail with reference to the drawings.

First, the present invention is based on the relationship among various characteristics of an induction motor, including winding resistance R₁, primary leakage reactance X₁, excitation resistance R_(o), terminal voltage V, etc., and will be described with reference to the equivalent circuit of FIG. 3 and the vector diagrams of FIGS. 4 and 5.

A secondary input P2 of an induction motor is found in accordance with

    P.sub.2 =E.sub.1 ×I.sub.1 ×cos θ.sub.2   (1)

Letting φ represent excitation flux and letting ω_(o) denote the excitation frequency, an induced electromotive force E₁ is expressed by

    E.sub.1 =dφ/dt=ω.sub.o ×φ              (2)

The secondary current is expressed by ##EQU1## The power factor is obtained from ##EQU2## Substituting Eqs. (2), (3), (4) into Eq. (1) gives us ##EQU3## Secondary core loss P₂ r is obtained from

    P.sub.2 r=I.sub.2.sup.2 R.sub.2 =S×P.sub.2           (6)

The output P_(m) of the motor is expressed by

    P.sub.m =P.sub.2 -P.sub.2 r×(1-S)P.sub.2             (7)

Letting T_(m) represent the motor torque and ω_(m) the rotational speed of the motor, we have

    Pm=ω.sub.m ×T.sub.m                            (8)

The torque T_(m) of the motor can be written as follows using Eqs. (1), (2), (7) and (8): ##EQU4## Note that ω_(s) represents the slip frequency.

Next, as shown in FIG. 5, the φ-axis component (excitation flux component) and E₁ -axis component (component in the direction of the electromotive force) of I_(o) and I₂ are obtained as I_(oM), I_(o) ω, and I_(2M), I₂ ω, respectively, as follows: Letting L_(o) represent the excitation impedance, we have

    φ=L.sub.o ×I.sub.o

so that

    I.sub.oM =(1/L.sub.o)×φ                          (10)

I_(o) ω is the core loss component, and we let K represent a constant of proportion. This gives us

    I.sub.o ω=K×ω.sub.o ×φ         (11)

as an approximation.

Further, transforming Eq. (9) gives us

    I.sub.2 ω=I.sub.2 ×cos θ.sub.2 =T.sub.m /φ(12)

The φ-axis component I_(2M) of the secondary current is obtained as follows: ##EQU5## We obtain I₂ ω from Eqs. (2), (3), (4), (12) as follows: ##EQU6## where X₂ =ω_(o) ×L₂ and ω_(s) =S×ω_(o)

Dividing both sides by φ gives us

    I.sub.2 ω/φ=(ω.sub.s ×R.sub.2)/[(R.sub.2.sup.2 +(ω.sub.s L.sub.2).sup.2 ]                          (14)

With the foregoing serving as a premise, we will now describe a preferred embodiment, in accordance with the invention, of obtaining the primary current and slip frequency conforming to a torque command.

In general, the maximum value of excitation flux φ is decided by using the maximum excitation current capable of flowing in the motor, or the maximum voltage capable of being impressed upon the motor. In actual control of a motor, there are cases where excitation is weakened when the load is small in order to reduce excitation noise. In the following explanation, however, it will be assumed that the flux command gives the maximum value.

When the torque command T_(m) and excitation flux command φ are applied, I_(oM) is found from Eq. (10), I_(o) ω from Eq. (11), and I₂ ω from Eq. (12).

Next, the slip frequency ω_(s) is decided in accordance with Eq. (14) using I₂ ω and φ. However, since it is difficult to calculate ω_(s) by a CPU or the like in a short period of time, values of ω_(s) are found in advance with respect to values of I₂ ω/φ, ω_(s) is stored in memory as a data table, and the value of I₂ ω/φ is stored in memory as a data address.

Thus, if I₂ ω and ω_(s) are found, I_(2M) can be obtained from Eq. (13).

From the foregoing, the φ-axis component I₁ (φ) of the primary current I₁ can be obtained from:

    I.sub.1 (φ)=I.sub.oM +I.sub.2M                         (15)

and the E₁ -axis component I₁ (E₁) of the primary current I₁ can be obtained from:

    I.sub.1 (E.sub.1)=I.sub.o ω+I.sub.2 ω          (16)

The primary current thus obtained is a current command, which takes all characteristics of the induction motor into account, for obtaining a linear output torque with respect to a torque command.

Further, I₁ (φ), I₁ (E₁) correspond to an excitation current component and load current component, respectively, in vector control of the induction motor.

The specifics of the present invention will now be described with reference to the schematic diagram of FIG. 1 and flowchart of FIG. 2.

(1) A velocity command ω_(c) and the induction motor rotational speed ω_(m), which is obtained by a tachogenerator TG coupled to the output shaft of the motor M, are applied to a comparator "a", and the torque command T_(m) is calculated by performing the arithmetic operation

    T.sub.m =K.sub.1 (ω.sub.c -ω.sub.m)+K.sub.2 ∫(ω.sub.c -ω.sub.m)dt

In vector control, a calculation method in which a calculation is performed as indicated by the above equation using a velocity deviation, the result of the calculation is regarded as a torque command and is commonly known; hence, a description of the foregoing is omitted.

(2) Excitation flux φ on a flux characteristic curve, as shown in FIG. 1 is decided from the excitation frequency ω_(o).

(3) The φ-direction component I_(oM) of excitation current I_(o) is obtained from the excitation flux φ as shown below and in FIG. 1.

    I.sub.oM =(1/L.sub.o)×φ

This is applied to an adder d₂.

(4) The E₁ -direction component of I_(o) is obtained from φ and ω_(o) in the form

    I.sub.o ω=K.sub.3 ×ω.sub.o ×φ

Where K₃ is a constant of proportionality and I_(OW) is applied to an adder d₁ (see FIG. 1).

(5) The E₁ -direction component of the secondary current I₂ is obtained in the form

    I.sub.2 ω=(T.sub.m /φ)

by the divider b₁.

(6) Using the I₂ ω and excitation flux command φ that have been obtained, the divider b2 divides I₂ ω by φ. Then, a slip frequency ω_(s) corresponding to the data address I₂ ω/φ, which is the result of the calculation, is obtained from a data table stored in memory beforehand.

Slip frequency W_(s) is then applied to adder d₃ along with motor rpm W_(m) and the sum is input to a 2-3 phase converter B.

(7) I₂ ω and ω_(s) are applied to a multiplier c₂, and the product is multiplied by (L₂ /R₂). The φ-direction component of the secondary current is obtained in the form

    I.sub.2M =(L.sub.2 /R.sub.2)×I.sub.2 ω×ω.sub.s

This is applied to the adder d₂.

(8) The φ-direction component of I₁ is obtained in the form

    I.sub.1 (φ)=I.sub.oM +I.sub.2M

by the adder d₂.

(9) The E₁ -direction component of I₁ is obtained in the form

    I.sub.1 (E.sub.1)=I.sub.o ω+I.sub.2 ω

by an adder d₁.

(10) Two orthogonal phases of the primary current command I₁ are obtained from a current calculating circuit A and applied to a 2-3 phase converter circuit B.

(11) Command currents Iu, Iv, Iw in the respective phases of the induction motor M obtained from the 2-3 phase converter circuit B are applied to a current controller C. The actual currents of the respective phases of the induction motor are sensed by CTu, CTv, CTw and compared with the command currents, upon which predetermined control is performed by the current controller C.

In accordance with the system for controlling the torque of an induction motor of the present invention, control is performed in such a manner that a linear output torque is obtained with respect to a torque command. This enables the output torque of the induction motor to be accurately judged at low cost.

Although certain preferred embodiments have been shown and described, it should be understood that many changes and modifications may be made therein without departing from the scope of the appended claims. 

What is claimed is:
 1. An induction motor torque control system for linearly controlling the output torque of an induction motor in response to a torque command, comprising:(a) means for deciding a torque command (Tm) for the induction motor, and an excitation flux (Φ) from an excitation frequency (Wo); (b) means for deciding an excitation flux component (Iom) of excitation current (Io) from the excitation flux (Φ); (c) means for deciding a component (Iow), in the direction of an electromotive force (E1), of the excitation current (Io) from the excitation flux (Φ) and excitation frequency (Wo); (d) means for deciding a component (I2w), in the direction of the electromotive force (E1), of a secondry current (I2) of the induction motor from said torque command (Tm) and excitation flux (Φ); (e) means for deciding a slip frequency (Ws) from the component (I2w), in the direction of the electromotive force (E1), of said secondary current (I2) and the excitation flux (Φ); (f) means for deciding an excitation flux component (I2m) of the secondary current (I2) from the component (I2w), in the direction of the electromotive force (E1), of the secondary current (I2) and from the slip frequency (Ws); and (g) means for obtaining an excitation flux component (I1(Φ)) of the primary current (I1) of the induction motor from the sum of the excitation flux components (Iom) and (I2m) of said excitation current (Io) and the secondary current (I2), and a component (I1 (E1)) in the direction of the electromotive force (E1) of the primary current (I1) of the induction motor from the sum of the components (Iow) and I2w) in the direction of the electromotive force (E1) of said excitation current (Io) and secondary current (I2), and for limiting a primary current command for the induction motor upon combining these components.
 2. An induction motor torque control system according to claim 1, wherein said slip frequency (Ws) deciding means decides the slip frequency (Ws) from a data table in which the data address is a value obtained by dividing the component (I2w), in the direction of the electromotive force (E1), of the secondary current (I2) by the excitation flux (Φ). 